var mergeTwoLists = function(list1, list2) {
    if(list1 === null) {
        return list2
    }
    if(list2 === null) {
        return list1
    }
    if(list1.val < list2.val) {
        list1.next = mergeTwoLists(list1.next,list2)
        return list1
    }else {
        list2.next = mergeTwoLists(list1,list2.next)
        return list2
    }
};

